The hydrodynamic models assuming equilibrated spin degrees of freedom successfully explained these observations of global spin-polarization but failed to show similar success in the case of longitudinal spin-polarization [1]}, [2]}. This indicated different possible origins of spin-polarization, which then led to the belief that the spin degrees of freedom in the transverse plane may not achieve equilibration at the time of freeze-out [3]}, and consequently, we may require dissipative theories of hydrodynamics.
